The invention relates to the overall field of ignition control in motor vehicle engines. It deals more particularly with the protecting of exhaust pipes, which are sensitive to overheating.
Most modern motor vehicles use, to reduce polluting emissions, exhaust systems of the catalytic type, which receive hot gases from the engine via the exhaust pipe.
These catalytic systems are particularly sensitive to excessive overheating, which destroys the chemical properties of the catalysts.
Now, it has been found that, for the phases of operation of the engine during which there is no desire to produce power, the ignition angle (often denoted IGA) for nominal ignition passes to a minimum ignition angle (which is close to stalling speed) and there is therefore a search to regulate the ignition event so that it takes place just early enough to burn the air/fuel mixture in the pistons, but with a combustion which is slightly offset in time (in comparison to normal operation) and produces no power.
This delayed combustion gives rise to a great dissipation of heat in the exhaust pipe. It is therefore essential to make sure that the temperature reached TEG (temperature of exhaust gas) can under no circumstances reach a limit which would cause the catalyst to be destroyed.
Methods for controlling temperature in exhaust pipes which employ real-time measurement of the temperature at a point on the pipe and possible injection of excess fuel into the mixture introduced in the pistons are known, this unburnt fuel (unburnt because it is in excess in the mixture) being intended solely to cool the exhaust pipe.
As will be understood, such a system is not very economical with fuel, and devices have been proposed for reinjecting the mixture leaving the catalytic system into the carburetor with a view to reducing this loss of unburnt fuel. Clearly these devices are at once complex, difficult to regulate, and expensive.
In general, temperature protection of the exhaust pipe is also obtained by keeping a margin between the minimum ignition advance to which the engine is set and the ignition advance that corresponds to stalling speed. In this way, the dissipation of heat into the pipe is reduced and this then affords protection against temperature. However, it is clear that this solution is barely satisfactory, in that it maintains the production of a certain amount of power by the engine.
The present invention therefore proposes a novel method for temperature-protecting an exhaust pipe.
According to the invention, the temperature protection method for a motor vehicle exhaust pipe comprises steps of determining a minimum ignition angle from several possible values, including a stalling-speed angle, on the basis of predetermined logic and of engine operating parameters.
As a preference, the minimum ignition angle may be set down as low as the stalling speed for periods of time in which certain engine parameters lie within a determined range of values.
The description and the drawings which follow will make it possible to gain a better understanding of the objects and advantages of the invention. Clearly, this description is given solely by way of example, and is not in any way limiting.